Below absolute zero reached in quantum gas

Nature : Absolute zero corresponds to the theoretical state in which the average energy of a system of particles is zero. During the normal state of a gas, the majority of the particles are at energies near the average, with just a few at higher energy levels. Theorists predicted in the 1950s that if a gas could be created in which the situation was reversedâmdash;the majority of the particles had higher energy levelsâmdash;then the temperature could drop to below absolute zero. Ulrich Schneider, with Ludwig-Maximilians University in Munich, and his colleagues appear to have done just that. Using lasers and magnetic fields, the researchers arranged a stable lattice structure out of a quantum gas of potassium atoms. Quickly adjusting the magnetic field caused the atoms to attract rather than repel each other, and they shifted from their lowest-energy state to a high-energy state. Normally, that would cause the lattice to collapse, but the researchers used the lasers to make it too difficult for the atoms to leave their positions. The result is a gas that has a temperature just a few billionths of a degree below absolute zero. The experiment opens the way to potential stable states of exotic materials, and the theoretical behavior of other systems at sub-absolute-zero temperatures may provide some answers about cosmological phenomena such as dark energy.